The present invention relates to a process for manufacturing a sodium perborate monohydrate in the form of granules and to the granules thus obtained.
There is a market for sodium perborate monohydrate on the one hand because of its elevated active oxygen content (theoretical value 16.03% corresponding to the formula NaBO.sub.3.H.sub.2 O), and on the other hand because it is stable at a higher temperature than sodium perborate tetrahydrate. It is thus used in special washing powders, in solid compositions for bleaching and in different particular cleaning agents.
Up to now, the usual process for manufacturing sodium perborate monohydrate has involved the dehydration of the tetrahydrated product. Various methods of dehydration are noted in the literature, for example, fluidized bed processes such as that described in Belgian Patent No. 718,160 filed July 17, 1968.
These processes have the disadvantage of requiring two successive operations, one leading to the manufacture of sodium perborate tetrahydrate, the other consisting of the dehydration of this product. Moreover, the active oxygen losses are relatively high in view of the accumulation of the losses in the manufacture of sodium perborate tetrahydrate together with those observed during the super-drying of the product. Finally, the product obtained is porous and friable.
A one stage process for manufacturing sodium perborate with an elevated active oxygen content by reaction of a boron source (boric acid, borax or metaborates), sodium hydroxide and hydrogen peroxide in aqueous solution was the object of a U.S. Pat. No. 2,937,998 issued May 24th, 1960.
This process has a number of disadvantages. Its technology is complicated. Also, the product obtained is friable and has a very fine granulometric structure, which means that a subsequent agglomeration operation is necessary in order to obtain granules of a size which is compatible with those of the other constituents of washing powders.
In Belgian Patent No. 847,617, filed on Oct. 25th, 1976, and corresponding to U.S. patent application Ser. No. 737,686, filed on Nov. 1st, 1976, process described for the manufacture of sodium perborate monohydrate in the form of granules resistant to abrasion, according to which an aqueous solution containing hydrogen peroxide and an aqueous solution containing sodium metaborate are simultaneously introduced into a fluidized bed dryer containing seeds of smaller dimensions than the granules to be obtained, and the excess water present in the said aqueous solution is evaporated by means of a fluidizing gas which is introduced into the fluid bed dryer.
Although the rate of dissolution in water of the sodium perborate monohydrate granules thus obtained according to Belgian Patent No. 847,617, is relatively high, it is occasionally insufficient for certain applications.